Field of the Invention
The present invention relates to a lens unit including a zoom function which changes an angle of field by changing a focal length. In particular, the present invention relates to a zoom lens unit used for an imaging optical system in a TV camera, video camera, or so-called digital camera, which obtains image data of a subject, and which is especially suitable for a monitoring video camera. The present invention also relates to an imaging device using such a zoom lens unit as an imaging optical system.
Description of the Related Art
There are several types of zoom lens units for a TV camera. There is a zoom lens unit for a TV camera including a first lens group with positive refractive power, a second lens group with negative refractive power, which moves when changing a magnification, a third lens group which moves when changing a magnification, and a fourth lens group which is fixed when changing a magnification. In this type of zoom lens unit, the second lens group plays a role as variator lenses for changing a magnification and the third lens group plays a role as compensator lenses for correcting a change in image surface position with a change in magnification, or both of the second and third lens groups play roles of the variator lenses and compensator lenses. The first lens group includes a focus function, and the fourth lens group plays a leading role in imaging as a master group.
A monitoring TV camera or monitoring video camera performs imaging with sensitivity in a wavelength range including a near-infrared light range of about 900 nm or below. Such a monitoring TV camera or monitoring video camera is configured to obtain an accurate color image only with visible light by cutting near-infrared light during the day with a sufficient light volume, to obtain a light volume by transmitting all light from a visible light range to a near-infrared light range in bad weather or twilight, and to project or illuminate infrared light at a wavelength of about 850 nm toward a dark side during the night. The zoom lens unit used for a photographing lens unit is therefore required to have corrected chromatic aberration not only in a visible light range but also in a near-infrared light range. When the chromatic aberration is not corrected in the wavelength range including the near-infrared light range, the zoom lens unit is required to refocus on the occasion of switching between visible light and near-infrared light, and the zoom lens unit cannot obtain sufficient resolving power in a case where the zoom lens unit transmits all light from the visible light range to the near-infrared light range.
To preferably correct chromatic aberration in the above-described zoom lens unit, a method of reducing secondary spectrum with the use of a positive lens made of an anomalous low dispersion glass such as OHARA S-FPL51 or OHARA S-FPL53 in the first lens group having a high axial marginal light beam height on the telephoto side and in the fourth lens group having a high axial marginal light beam height on the wide-angle side. However, the anomalous low dispersion glass has a large negative temperature coefficient regarding a refractive index and a large positive linear expansion coefficient. For this reason, these large coefficients are added, refractive power as a lens significantly changes with a temperature change, and a change in imaging position (out of focus) of an entire optical system easily occurs.
JP H 08-297244A (Patent Literature 1), JP 2013-033242A (Patent Literature 2), and JP 2008-241884A (Patent Literature 3) describe the above-described zoom lens unit as a conventional example.
The zoom lens unit disclosed in Patent Literature 1 is configured to control a change in imaging position with a temperature change by devising the material of the negative lens in a lens unit in addition to a focus lens unit while using a positive lens made of an anomalous low dispersion glass in the lens unit. However, such a zoom lens unit is not sufficient as a monitoring zoom lens unit since the chromatic aberration is not corrected in a wavelength range including a near-infrared light range.
The zoom lens unit disclosed in Patent Literature 2 is configured to control out of focus of an optical system with a temperature change by devising the configuration of the fourth lens group, and/or the material and the refractive power of respective lenses in the third lens group. However, such a zoom lens unit requires an impractical material as a normal optical glass. Such a zoom lens unit is not sufficient as a monitoring zoom lens unit since the chromatic aberration is not corrected in a wavelength range including a near-infrared light range.
The zoom lens unit disclosed in Patent Literature 3 is configured to correct the chromatic aberration in a wavelength range including a near-infrared light range with the use of an anomalous low dispersion glass in the first and fourth lens groups. However the zoom lens unit is not configured to control a change in imaging position with a temperature change.